Application of Inkjet Printing Technology toProduce Test Materials of 1,3,5-Trinitro-1,3,5Triazcyclohexane for Trace Explosive Analysis

Eric Windsor,*,† Marcela Najarro,† Anna Bloom,† Bruce Benner, Jr.,† Robert Fletcher,†

Richard Lareau,‡ and Greg Gillen†

Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg,Maryland 20899, and Transportation Security Laboratory, Science and Technology Directorate, Department ofHomeland Security, Atlantic City, New Jersey

The feasibility of the use of piezoelectric drop-on-demandinkjet printing to prepare test materials for trace explosiveanalysis is demonstrated. RDX (1,3,5-trinitro-1,3,5 tri-azcyclohexane) was formulated into inkjet printable solu-tions and jetted onto substrates suitable for calibrationof the ion mobility spectrometry (IMS) instruments cur-rently deployed worldwide for contraband screening.Gravimetric analysis, gas chromatography/mass spec-trometry (GC/MS), and ultraviolet-visible (UV-vis) ab-sorption spectroscopy were used to verify inkjet printersolution concentrations and the quantity of explosivedispensed onto test materials. Reproducibility of the inkjetprinting process for mass deposition of the explosive RDX(1,3,5-trinitro-1,3,5 triazcyclohexane) was determined tobe better than 2% for a single day of printing and betterthan 3% day-to-day.

With the threat of global terrorism on the rise, the ability todetect trace levels of explosives has become an issue of criticalnational importance. This is especially true at screening locationsincluding airports, seaports, U.S. embassies, and other governmentfacilities. Although analytical techniques exist to detect quantitiesof explosives at or below the nanogram (ng) level, most methodscannot handle the high-throughput sampling demands that existat screening locations.1-3 Ion mobility spectrometry (IMS) is ananalytical technique capable of rapid (≈10 s) trace level explosiveanalysis. Commercially available IMS systems are rugged, por-table, and capable of operating at atmospheric pressure. For thesereasons, IMS instruments have been widely deployed. It isestimated that approximately ten thousand IMS instruments arein use for trace-level explosive screening.4 For this work, we definetrace detection as chemical analysis of explosive residues atmasses between 0.1 and 100 ng.

In a typical screening implementation, personnel wipe baggageor cargo surfaces such as luggage handles or package labels witha “trap” composed of cloth, paper, or polytetrafluoroethylene(PTFE)-coated materials. Explosive particles are removed fromthe wiped surfaces and collected onto the traps. Traps are thenintroduced into the IMS instrument where the explosive particlesare vaporized at temperatures exceeding 200 °C. The explosivevapor is then introduced into the ion source region where it isionized via adduct formation with Cl- reactant ions created byelectron impact of a chlorinated reactant gas such as hexachlo-roethane with a � particle emission from a 63Ni source. Afterionization, the adduct ions are injected into a drift tube wheretheir atmospheric gas phase mobility is determined by theirtime-of-flight in a weak electric field. Ions travel through thedrift tube at different rates depending on their size, shape, andcharge, and the measured drift time is compared to a referencelibrary of known explosives for identification.

Test materials (traps) containing known and reproducible(typically nanogram) quantities of explosives are critical forcalibrating IMS instruments and ensuring that they are operatingproperly at deployment locations. Desired properties for such testmaterials include the following: high precision and accuracy, alarge dynamic range in concentration, and scalability to allow highthroughput for rapid and inexpensive production to accommodatethe large numbers of deployed IMS systems. A variety of differentexplosive test materials are needed to provide flexibility to respondto the large and often changing number of threats. Using inkjetprinting technology to produce trace explosive test materialsrepresents one promising approach to meeting all of theserequirements.

Piezoelectric drop-on-demand inkjet printing is a versatilemethod for the quantitative delivery of microvolumes of solutions.5

Inkjet printing offers noncontact, high-throughput deposition ofprecise quantities of materials. The reproducibility of optimizedinkjet printers has been reported to be better than 2% relativestandard deviation for day-to-day measurements of dispensedvolumes.6 Large dynamic ranges in deposited analyte concentra-tion (105) are achieved simply by varying the number of dropsprinted. Over the past decade, inkjet printing technology has

* To whom correspondence should be addressed. Phone: (301) 975-3930.E-mail: eric.windsor@nist.gov.

† National Institute of Standards and Technology.‡ Department of Homeland Security.

(1) Steinfeld, J.; Wormhoudt, J. Annu. Rev. Phys. Chem. 1998, 49, 203.(2) Yinon, J. Forensic and Environmental Detection of Explosives; John Wiley:

Chichester, NY, 1999.(3) Moore, D. S. Rev. Sci. Instrum. 2004, 75 (8), 2499.(4) Eiceman, G. A.; Stone, J. A. Anal. Chem. 2004, 76 (21), 390A.

(5) Lee, E. Microdrop Generation; CRC Press: Boca Raton, FL, 2003.(6) Englmann, M.; Fekete, A.; Gebefugi, I.; Schmitt-Kopplin, P. Anal. Bioanal.

Chem. 2007, 388, 1109.

Anal. Chem. 2010, 82, 8519–8524

10.1021/ac101439r Not subject to U.S. Copyright. Publ. 2010 Am. Chem. Soc. 8519Analytical Chemistry, Vol. 82, No. 20, October 15, 2010Published on Web 09/28/2010

been widely applied to diverse applications ranging from theprinting of DNA microarrays to dispensing metallic solder forinterconnects in the electronics industry.5,7,8 If explosives canbe printed in a reproducible manner, then test materials can beproduced rapidly and inexpensively. This paper demonstrates thefeasibility of the use of drop-on-demand inkjet printing technologyto produce RDX (1,3,5-trinitro-1,3,5 triazcyclohexane) test materi-als for trace level explosive analysis.

EXPERIMENTAL SECTIONStock RDX Solution. The high explosive RDX was purchased

as standard solutions consisting of individually packaged ampules(approximately 1 mL) containing the explosive dissolved in eitheracetonitrile or methyl alcohol at nominal concentrations of 1000µg/mL (Restek, Bellefonte, PA; Supelco, Bellefonte, PA; andCerilliant, Round Rock, TX). (Certain commercial equipment,instruments, or materials are identified in this report to specifyadequately the experimental procedure. Such identification doesnot imply recommendation or endorsement by the NationalInstitute of Standards and Technology nor does it imply that thematerials or equipment identified are necessarily the best availablefor the purpose.) Isotopically labeled RDX (13C3-15N3-RDX, 99%,Cambridge Isotope Laboratories, Andover MA) at a nominalconcentration of 1000 µg/mL was obtained for use as aninternal standard for gas chromatography/mass spectrometry(GC/MS) quantitative analysis.

We have found that typical solvents used to dissolve explosivesare not optimal for printing. The rheological properties of solventssuch as acetonitrile, methanol, and tetrahydrofuran are outsidethe typical range needed for stable operation of our printersystems (kinematic viscosity 1 × 10-6-30 × 10-6 m2/s andsurface tension 0.02-0.06 N/m).5 To achieve printer stability,explosives were reconstituted in isobutyl alcohol (IBA). Re-constitution was achieved by transferring a known volume ofexplosive solution to a 30 mL inkjet printer bottle, allowing thesolvent to evaporate, and then redissolving the remainingexplosive residue in IBA. Solubility of explosives in IBA is lessthan the solubility in the original solvents. Therefore, themaximum explosive concentrations of the reconstituted inkjetprinter solutions are often <200 µg/mL. All inkjet printer

solutions are filtered prior to use using 0.2 µm PTFE syringefilters to minimize the possibility of clogging the inkjet printersystem.

Inkjet Printing. Test materials were produced using a JetlabII (MicroFab, Plano, TX) inkjet printer system. The Jetlab II is adrop-on-demand inkjet printing system with precision X, Y, Zmotion control, drop ejection drive electronics, pressure control,and a drop visualization system. The print head consists of a glasscapillary tube with a 50 µm diameter orifice surrounded by apiezoelectric crystal. Voltage pulses (≈20-60 V; rise time ) 3-5µs; dwell time ) 20-40 µs; fall time ) 3-5 µs) applied to thepiezo crystal cause it to expand and then contract around thecapillary producing an acoustic wave that propagates throughthe printing fluid in the capillary tube. When an acoustic wave ofsufficient energy reaches the orifice, a microdroplet is ejected.Stroboscopic illumination and a charged coupled device (CCD)camera are used to visualize droplet ejection (Figure 1). For stabledroplet ejection, the printer conditions are tuned by visuallyobserving the ejected microdrops while adjusting the voltage pulseparameters and the backfill pressure on the fluid in the capillary.Optimal jetting is generally found using conditions that give thehighest microdrop velocity without satellite droplet formation.Printing was performed at a frequency of 2000 Hz with a dropletejection velocity of ≈2 m/s and an average drop volume of 116pL (diameter ) 60.5 µm). Detailed studies recently conducted inour laboratory have used advanced gravimetric approaches torigorously evaluate the effect of voltage pulse parameters, backfillpressure, and drop ejection velocity on printer stability, reproduc-ibility, and droplet size with the goal of developing optimalconditions for future printing studies.9

When printing, multiple traps are held in a custom designedsample holder. The print head remains fixed while the precisionstage moves from trap-to-trap. We print array patterns containingsmall numbers of drops (1-10) at each array element rather thana single deposit at one point. This assures rapid evaporation times,minimal bleed-through of solution to the backside of the trap, andpresents a more uniform sample to the desorber system of theIMS. Sample throughput is limited by the time required toreposition the stage at each trap. Arrays can be printed using threedifferent methods: drop-on-demand, print on the fly, and print onthe fly burst mode. In drop-on-demand mode, the stage moves indiscrete increments from array point to array point dispensingone or more drops at each point. With print on the fly, a single

(7) Theriault, T. P.; Winder, S. C.; Gamble, R. C. DNA Microarrays-APractical Approach; Schena, M., Ed.; Oxford University Press, Oxford,1999; Chapter 6.

(8) Hayes, D. J.; Wallace, D. B.; Cox, W. R. Proc. IMAPS Int. Conf. High DensityPackag. MCMs 1999, 242. (9) Verkouteren R. M.; Verkouteren, J. Anal. Chem. 2009, 81, 8577.

Figure 1. Drop generation from Jetlab II piezoelectric ink jet printer. Drops are shown at (a) 100 µs after drop generation and (b) 150 µs afterdrop generation. Each division on scale ) 32 µm.

8520 Analytical Chemistry, Vol. 82, No. 20, October 15, 2010

drop is dispensed at each array element while the stage iscontinuously moving. In print on the fly burst mode, the stagemoves continuously but multiple drops are dispensed at each arraypoint. Print on the fly burst mode significantly improves samplethroughput. Using the drop-on-demand mode and dispensing onedrop at each array element, it took 90 min. With print on the flymode, dispensing one drop at each array element, this time wasreduced to 2 min 48 s, and when print on the fly burst mode isused, dispensing five drops per burst, it took only 24 s. Onepotential drawback of printing in burst mode is a slight loss ofpositional accuracy due to spreading or elongation of array pointelements. If positional accuracy is critical, then printing shouldbe performed using the single drop mode rather than burst mode.For the present application, this is not thought to be of significantconcern.

Substrate (Trap) Materials. Visualization of inkjet printeddrop patterns on traps is a valuable tool especially during thedevelopment of new explosive “ink” solutions. Visualizing printedarray patterns is a useful technique for determining whether theprinter is performing stably and reproducibly. Drop patternvisibility is achieved by adding fluorescent dye to the printersolution and using substrates that allow visibility such as inkjetpaper and membrane filter materials composed of cellulose estersor polycarbonate (Figure 2a).

Although these substrates are useful for inkjet purposes, theycannot be used for the production of trace explosive test materialsbecause they are not thermally stable at the elevated temperaturesused during IMS analysis. Therefore, to produce explosive testmaterials for IMS, it is necessary to print onto thermally stabletraps with no volatile compounds that interfere with the IMSanalysis. Traps produced by IMS manufacturers (commerciallycalled swipes, filters, or swabs) fulfill these requirements andmake good substrates for trace level explosive test materials. Theonly disadvantage of the manufacturers traps is that inkjet printedpatterns are difficult to see due to a high fluorescent backgroundin these materials.

Through trial and error testing, it was determined that ashlessgrade filter papers (Whatman 41, Middlesex, UK) were compatiblewith IMS testing temperatures while also allowing adequatevisualization of inkjet printed patterns (Figure 2b). A blank IMSanalysis performed on the filter paper was observed to deplete

dopant ions in IMS instruments more rapidly than blank analysesperformed on manufacturer’s traps. This was most likely due tovolatile components on the filter paper. Pretreatment of the filterpaper using a solvent wash (acetone soak overnight ≈ 15 h) anda thermal treatment (oven at 115 °C overnight) was observed tominimize IMS interference. Therefore, we routinely perform thepretreatments prior to making explosive test materials. Treatedfilter papers were used for all analyses in this study. The additionof dye to the inkjet printer solution was used during methoddevelopment and for trouble shooting purposes. To minimize anypotential interference with the chemical analysis of the explosives,dye was not used during the production of the actual test materials.An example of a typical inkjet printed pattern is shown in Figure2. The printer solution was isobutyl alcohol with 25 µg/mLfluorescein dye added for pattern visibility. An array patterncontaining 400 drops (20 × 20 spots containing 1 drop each) wasprinted with a drop spacing of 0.3 mm. In Figure 2a, the patternwas printed onto a mixed cellulose ester membrane filter [(0.05µm pore size), Millipore, Bedford, MA]. Figure 2b shows the samepattern printed onto the ashless grade filter paper.

Gravimetry. Mass measurements were made using a MettlerAE 240 analytical balance, a Mettler AT21 Comparator microbal-ance (Toledo, OH), and a Sartorius SE2-F ultra microbalance(Goettingin, Germany). The repeatability of each balance wasdetermined by making repeated mass measurements of thecontainers used in this study. Balance accuracy was determinedby measuring the mass of calibration weights that were of similarmass to the containers weighed in this study.

Gas Chromatography/Mass Spectrometry. Gas chroma-tography/mass spectrometry analyses were performed using a1200 Varian (Palo Alto, CA.) GC/MS. Aliquots of 1 µL wereisothermally injected (by autosampler) at 150 °C using a split valveinjector with a split ratio of 50/1. The GC column (12 m × 0.22mm HT-5-SGE, Austin, TX) was operated at a constant heliumcarrier flow of 6 mL/min and held at 130 °C for 2 min, followedby a 20 °C/min increase to 160 °C with a 1 min hold. The massspectrometer was operated in the negative ion chemical ionization(NCI) mode with methane as the reagent gas. For RDX quantita-tive analysis, an isotopically labeled RDX (13C3-15N3-RDX, 99%)solution was added as an internal standard (Cambridge IsotopeLaboratories, Andover, MA). Ions were monitored at mass/

Figure 2. (a) Fluorescence micrograph of an array pattern of single drops (20 × 20, 0.3 mm spacing between points) printed on a mixedcellulose ester membrane filter (0.05 µm pore size). (b) Same array printed on quantitative grade ashless filter paper. Fluorescein dye wasadded to the printer solution to allow drop visibility.

8521Analytical Chemistry, Vol. 82, No. 20, October 15, 2010

charge (m/z) of 102 and 129 for RDX and at m/z of 104 and135 for isotopically labeled RDX at a fixed electron multipliervoltage of 1200 V. The 129 m/z (RDX) and 135 m/z (13C3-15N3-RDX) fragment ions were selected for quantitative analysisbased on their greater relative abundance. For quantitativepurposes, a calibration curve was produced from five standardsolutions ranging in RDX concentration from 0.1 to 100 µg/mL. Each standard solution was prepared by mixing a knownmass of RDX standard solution with a known quantity of theisotopically labeled RDX internal standard. These calibrationsolutions were used to determine instrument response factorsthat were then used to quantify the amount of RDX dispensedfrom the inkjet printer.

Inkjet-produced samples were prepared for GC/MS analysisby inkjet printing into deactivated glass inserts placed within GC/MS sample vials. When printing into inserts, fluid was sometimesobserved on the sides of the inserts indicating that the droplettrajectory was not always straight down into the bottom of theinsert. To ensure that no explosive was lost prior to GC/MSanalysis, the inkjet nozzle was inserted approximately halfway intothe insert, the drops were dispensed, and then the nozzle wasremoved. The internal standard was added by pipet, and theinserts were filled with solvent to a final volume of 250 µL. Addingadditional solvent to recover explosive from the sides of the insertsrequired that a larger number of drops be printed to achieve thedesired target GC/MS concentrations. Each vial was tested sixtimes by GC/MS analysis, and the average from these six replicateanalyses was used as the final RDX concentration for the vial.

Ultraviolet/Visible Absorption Spectroscopy (UV/vis).UV/vis absorption spectroscopy measurements were performedusing a Shimadzu UV-1800 double beam spectrophotometeroperated in photometric mode. Five point RDX calibration curveswere made by taking aliquots from standard solutions containingthe explosive in acetonitrile (Restek, Bellefonte, PA; Supelco,Bellefonte, PA; and Cerilliant, Round Rock, TX). The acetonitrilewas evaporated, and the explosive residue was reconstituted inisobutyl alcohol and diluted to concentrations that spanned theinstruments linear dynamic range for absorbance. Absorbance wasmeasured using matched quartz cuvettes with a light path of 10mm. One cuvette was filled with 1 mL of the explosive standardsolution; the other functioned as a blank and was filled with 1 mLof isobutyl alcohol. Absorbance was then measured using wave-length scans from 230 to 300 nm.

Ion Mobility Spectrometry. Ion mobility spectrometry analy-ses were performed using the GC-IONSCAN 400B (SmithsDetection, Warren, NJ.) operated in the IONSCAN mode at adesorber temperature of 230 °C, a tube temperature of 114 °C,and a sampling time of 7 s. The instrument drift time is calibratedby means of an internal calibrant. All analyses were performed inthe negative ion mode using hexachloroethane gas to producethe reactant ions.

RESULTS AND DISCUSSIONTo validate the inkjet printing process for quantity of explosive

dispensed and process reproducibility, a test run through theentire printing process was performed using RDX as the explosive.Testing was performed over three consecutive days, allowing forthe determination of both within-day and day-to-day reproduc-ibility. RDX was selected because it is widely used in a variety of

explosives and is a major component of many plastic-bonded puttyexplosives. The remainder of this document discusses this RDXtest run during which the following test materials were produced.(1) One hundred and thirty (130) trap samples for IMS testingand calibration. Ten traps were prepared at each of the followingthirteen RDX quantities: 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 15, 20, 40, 60,80, and 100 ng. (2) Thirty (30) vials for the gravimetric and GC/MS determination of explosive quantity and inkjet printer repro-ducibility. Vials were printed at six different target concentrationsfor GC/MS analysis: 0.5, 1, 5, 15, 35 and 50 µg/mL. Twelve vialswere printed on day one (two at each target concentration), sixvials on day two (one at each target concentration), and twelvevials on day three (two at each target concentration).

Starting Solution Concentration. The concentration of thestarting inkjet printer solution was determined by gravimetry andverified by two additional independent analytical techniques: GC/MS and ultraviolet-visible (UV-vis) absorption spectroscopy. Forgravimetry, an ampule containing 1 mL of standard RDX solutionwith starting concentration of 1000 ± 5 µg/mL was used. Thesolution was transferred to a previously weighed inkjet printerbottle (volume capacity 30 mL) that was then reweighed todetermine the mass of solution transferred. The cap was removed,and the acetonitrile was allowed to evaporate. A measured massof IBA (corresponding to 15.5 mL) was then added to the RDXresidue to form the starting inkjet printer solution. From themanufacturers certified concentration value and gravimetricmeasurements, the starting concentration of the printer solutionwas determined to be 64.4 ± 0.3 µg/mL (assuming negligible lossof RDX during solvent evaporation). (Uncertainty representspropagation of errors resulting from the uncertainty in the startingsolution concentration, the uncertainty in the balance readings,and the reproducibility of the balance.) Independent measure-ments of the concentration of this printer solution were deter-mined by GC/MS and UV-vis spectroscopy to be 62.9 ± 0.3 and62.8 ± 0.2 µg/mL, respectively. (Uncertainty is the standarddeviation from replicate analyses from a single sample.)

Quantity of Explosive Dispensed by Inkjet Printer. Bydetermining the mass of RDX per dispensed droplet, we were ableto produce test materials containing a range of different explosivequantities simply by printing different numbers of drops. RDXmass per drop was determined by dispensing preselected numbersof drops (2000 Hz) into GC bottles containing glass inserts forboth gravimetric and chemical analyses. The number of dropletsprinted was selected to target six different concentrations for GC/MS analysis: 0.5, 1, 5, 15, 35 and 50 µg/mL. After printing, thepreweighed bottles were capped immediately and reweighed todetermine the mass of the solution dispensed. Dividing solutionmass by the specific gravity of IBA and multiplying this quotientby the concentration of the printer solution yields the mass ofRDX dispensed by the inkjet printer. Dividing RDX mass by thenumber of drops dispensed gives the average mass of RDX perdrop. Gravimetrically determined mass for the six different targetconcentrations is listed in Table 1.

Gravimetric determinations were verified by GC/MS. For GC/MS analysis, an internal standard was added and the inserts werefilled with solvent (acetonitrile) to a final volume of 250 µL.Analyses were performed and the concentrations are listed in

8522 Analytical Chemistry, Vol. 82, No. 20, October 15, 2010

Table 1. For comparison with gravimetric analyses, GC/MSanalyses were converted to RDX mass by multiplying thedetermined concentration by the volume of solution in the insert(≈250 µL). Comparison of the two techniques shows differencesof less than 4% for all concentrations printed. The linear relation-ship between the number of drops dispensed and the RDXconcentration determined by GC/MS analysis is plotted in Figure3. Linear regression analysis of the data yields an r2 value of0.99998.

Potential Bias from Solvent Evaporation. Gravimetricmeasurements are potentially biased by not considering theevaporation of isobutyl alcohol while printing into bottles. Toinvestigate this bias, a comparison was done between the massof solvent dispensed into preweighed bottles (as previouslydescribed) and the mass of an identical quantity of solventdispensed into a bottle placed directly on the weighing pan of amicrobalance. The microbalance was interfaced with a computer,and mass readings were recorded every 0.5 s. The evaporationrate was determined by dispensing solvent into the bottle and thenmonitoring the mass loss over time. Drop mass was determinedby dispensing 50 000 drops into preweighed bottles that were thencapped and reweighed to determine the mass of solvent dispensed.Immediately thereafter, 50 000 drops were dispensed into thebottle on the microbalance. The maximum balance reading wasrecorded as the mass of the 50 000 drops. This mass was corrected

for evaporation and compared with the mass determined byprinting into bottles (evaporation not considered). This procedurewas repeated six times over an 8 h time period, and the resultsare recorded in Table 2. The average bias introduced by notconsidering evaporation in the mass measurements was deter-mined to be 1.2%.

Reproducibility. The gravimetric data collected over the 3day testing period was used to determine the reproducibility ofthe inkjet printer for producing trace level explosive materials.Within-day reproducibility was determined by analyzing drop massdata from the 12 bottles printed on day one and day three andthe six bottles printed on day two. Day-to-day reproducibility wasdetermined from drop mass calculations from all 30 bottles printedover the 3 day testing period. Results are summarized in Table 3.The within-day reproducibility for drop mass as measured by thepercent relative standard deviation ranged from 0.7% to 1.8%whereas the day-to-day reproducibility was 2.6%.

Inkjet Printed Samples and IMS Calibration. The sampletraps produced during the RDX test run were used to calibrate

Table 1. Comparison between Dispensed Mass of RDX Determined by Gravimetry and GC/MSa

target GC/MSconc. (µg/mL)

actual GC/MSconc. (µg/mL)

dispensed RDX mass(GC/MS)b (µg)

dispensed solutionmass (gravimetry)

(µg)dispensed RDX mass

(gravimetry)c (µg) differenced (%)drops

dispensedRDX mass per drop(gravimetry) (µg)

0.5 0.73 ± 0.02 0.179 ± 0.006 2.25 × 103 ± 3.9 × 101 0.181 ± 0.003 1.3 24 317 7.44 × 10-6 ± 3 × 10-8

1 1.44 ± 0.03 0.351 ± 0.006 4.55 × 103 ± 1.1 × 102 0.365 ± 0.009 3.8 48 634 7.51 × 10-6 ± 2 × 10-8

5 7.3 ± 0.2 1.77 ± 0.06 2.25 × 104 ± 8.9 × 102 1.80 ± 0.07 1.8 243 170 7.42 × 10-6 ± 2 × 10-8

15 14.6 ± 1.0 3.61 ± 0.19 4.56 × 104 ± 1.0 × 103 3.66 ± 0.08 1.4 486 340 7.53 × 10-6 ± 2 × 10-8

35 36.3 ± 1.7 8.93 ± 0.46 1.13 × 105 ± 3.1 × 103 9.09 ± 0.25 1.7 1 215 851 7.47 × 10-6 ± 2 × 10-8

50 51.2 ± 2.5 12.66 ± 0.58 1.59 × 105 ± 5.1 × 103 12.79 ± 0.41 1.1 1 729 147 7.40 × 10-6 ± 1 × 10-8

a Dispensed mass values are averages from five different vials. Uncertainties are the standard deviation of the five measurements. The concentrationvalues are the average from five different vials printed over a 3 day period. b Value is calculated by multiplying the GC/MS conc. by the volumeof solution in the insert (≈0.250 mL). c Value is calculated by dividing the dispensed solution mass by the specific gravity of IBA (0.8018) andmultiplying this value by the RDX concentration of the printer solution (64.4 µg/mL). d Value is calculated from the equation: (x1 - x2)/((x1 +x2)/2) × 100 where x1 ) gravimetric concentration and x2 ) GC/MS concentration.

Figure 3. Experimental calibration plot of the RDX concentrationdetermined by GC/MS analysis versus the number of drops dispensedby the ink jet printer. Data points represent the average of fivemeasurements. The uncertainty represents the standard deviation ofthe measurements.

Table 2. Bias in Drop Mass Resulting from NotCorrecting Gravimetric Results for Evaporationa

drop mass (ng)(uncorrected)

drop mass (ng)(corrected for evaporation) bias (%)

90.0 ± 0.2 90.4 0.489.8 ± 0.2 92.0 3.293.2 ± 0.2 94.3 1.192.5 ± 0.2 94.1 1.793.5 ± 0.2 94.8 1.390.9 ± 0.2 90.7 -0.3

a Bias is calculated by the formula: Bias ) (x2 - x1)/x2 × 100 wherex2 ) corrected mass, x1 ) uncorrected mass. The uncertainty in theuncorrected drop mass results from the propagation of uncertaintiesin the balance readings divided by the number of drops printed.

Table 3. Gravimetric Analysis for Reproducibility of theInkjet Printer

no.analyses

gravimetry drop mass (ng)

day averagestandarddeviation % RSD

1 12 93.6 0.7 0.72 6 96.0 1.7 1.83 12 90.7 1.6 1.83 day total

(combined result)30 92.9 2.4 2.6

8523Analytical Chemistry, Vol. 82, No. 20, October 15, 2010

the GC IONSCAN 400B IMS instrument (Figure 4). The detectorresponse for this calibration shows classic IMS behavior displayinga linear range at low concentrations (0.05-15 ng) transformingto a second linear range of lesser slope at higher concentrations(15- 100 ng).10,11

CONCLUSIONS

Explosives can be formulated into solutions that print repro-ducibly using piezoelectric drop-on-demand inkjet printing tech-nology. Printer reproducibility was determined by gravimetrictechniques to be better than 2% for within-day measurements andbetter than 3% for day-to-day comparisons. Gravimetry, GC/MS,and UV-vis absorption techniques were used to determine thequantity of explosive dispensed by the inkjet printer. Inkjet-printedtrace level explosive test materials were used to calibrate IMSinstruments at NIST. Explosive test materials are currentlyundergoing further field testing at several different federalagencies.

ACKNOWLEDGMENTFunding for this work was supplied by the Science and

Technology Directorate of the United States Department ofHomeland Security (DHS) through an interagency agreement.

SUPPORTING INFORMATION AVAILABLEFigure 1 showing three modes of printing. Figure 2 showing

a sheet of inkjet printed explosive test materials. This material isavailable free of charge via the Internet at http://pubs.acs.org.

Received for review June 1, 2010. Accepted September 7,2010.

AC101439R

(10) Smith, G. B.; Eiceman, G. A.; Walsh, M. K.; Critz, S. A.; Andazola, E.; Ortega,E.; Cadena, F. Field Anal Chem. Technol. 1997, 4, 213.

(11) Eiceman, G. A.; Karpas, Z. Ion Mobility Spectrometry, 2nd ed.; CRC Press:Boca Raton, FL, 2005.

Figure 4. Calibration of an IMS instrument (Smith’s GC IONSCAN400B) using inkjet generated test materials of RDX printed on treatedfilter paper substrates. Data points represent the average value offive measurements. Uncertainties are the standard deviation of thesemeasurements.

8524 Analytical Chemistry, Vol. 82, No. 20, October 15, 2010